Module
ActiveRecord::Transactions::ClassMethods

Transactions are protective blocks where
SQL statements are only permanent if they can all succeed as one atomic
action. The classic example is a transfer between two accounts where you
can only have a deposit if the withdrawal succeeded and vice versa. Transactions enforce the integrity of the
database and guard the data against program errors or database break-downs.
So basically you should use transaction blocks whenever you have a number
of statements that must be executed together or not at all.

For example:

ActiveRecord::Base.transaction do
david.withdrawal(100)
mary.deposit(100)
end

This example will only take money from David and give it to Mary if neither
withdrawal nor deposit raise an exception.
Exceptions will force a ROLLBACK that returns the database to the state
before the transaction began. Be aware, though, that the objects will
not have their instance data returned to their pre-transactional
state.

Different Active Record classes in a single transaction

Though the transaction class method is called on some Active Record class,
the objects within the transaction block need not all be instances of that
class. This is because transactions are per-database connection, not
per-model.

In this example a balance record is transactionally saved even
though transaction is called on the Account
class:

Account.transaction do
balance.save!
account.save!
end

The transaction method is also available as a model instance
method. For example, you can also do this:

A transaction acts on a single database connection. If you have multiple
class-specific databases, the transaction will not protect interaction
among them. One workaround is to begin a transaction on each class whose
models you alter:

Student.transaction do
Course.transaction do
course.enroll(student)
student.units += course.units
end
end

This is a poor solution, but fully distributed transactions are beyond the
scope of Active Record.

save and destroy are automatically wrapped in a transaction

Both save and destroy come wrapped in a
transaction that ensures that whatever you do in validations or callbacks
will happen under its protected cover. So you can use validations to check
for values that the transaction depends on or you can raise exceptions in
the callbacks to rollback, including after_* callbacks.

As a consequence changes to the database are not seen outside your
connection until the operation is complete. For example, if you try to
update the index of a search engine in after_save the indexer
won't see the updated record. The after_commit callback is
the only one that is triggered once the update is committed. See below.

Exception handling and rolling back

Also have in mind that exceptions thrown within a transaction block will be
propagated (after triggering the ROLLBACK), so you should be ready to catch
those in your application code.

One exception is the ActiveRecord::Rollback exception, which
will trigger a ROLLBACK when raised, but not be re-raised by the
transaction block.

Warning: one should not catch
ActiveRecord::StatementInvalid exceptions inside a transaction
block. ActiveRecord::StatementInvalid exceptions indicate that
an error occurred at the database level, for example when a unique
constraint is violated. On some database systems, such as PostgreSQL,
database errors inside a transaction cause the entire transaction to become
unusable until it's restarted from the beginning. Here is an example
which demonstrates the problem:

# Suppose that we have a Number model with a unique column called 'i'.
Number.transaction do
Number.create(i: 0)
begin
# This will raise a unique constraint error...
Number.create(i: 0)
rescue ActiveRecord::StatementInvalid
# ...which we ignore.
end
# On PostgreSQL, the transaction is now unusable. The following
# statement will cause a PostgreSQL error, even though the unique
# constraint is no longer violated:
Number.create(i: 1)
# => "PGError: ERROR: current transaction is aborted, commands
# ignored until end of transaction block"
end

One should restart the entire transaction if an
ActiveRecord::StatementInvalid occurred.

Nested transactions

transaction calls can be nested. By default, this makes all
database statements in the nested transaction block become part of the
parent transaction. For example, the following behavior may be surprising:

User.transaction do
User.create(username: 'Kotori')
User.transaction do
User.create(username: 'Nemu')
raise ActiveRecord::Rollback
end
end

creates both “Kotori” and “Nemu”. Reason is the
ActiveRecord::Rollback exception in the nested block does not
issue a ROLLBACK. Since these exceptions are captured in transaction
blocks, the parent block does not see it and the real transaction is
committed.

In order to get a ROLLBACK for the nested transaction you may ask for a
real sub-transaction by passing requires_new: true. If
anything goes wrong, the database rolls back to the beginning of the
sub-transaction without rolling back the parent transaction. If we add it
to the previous example:

User.transaction do
User.create(username: 'Kotori')
User.transaction(requires_new: true) do
User.create(username: 'Nemu')
raise ActiveRecord::Rollback
end
end

only “Kotori” is created. This works on MySQL and PostgreSQL. SQLite3
version >= '3.6.8' also supports it.

Most databases don't support true nested transactions. At the time of
writing, the only database that we're aware of that supports true
nested transactions, is MS-SQL. Because of this, Active Record emulates
nested transactions by using savepoints on MySQL and PostgreSQL. See dev.mysql.com/doc/refman/5.6/en/savepoint.html
for more information about savepoints.

There are two types of callbacks associated with committing and rolling
back transactions: after_commit and
after_rollback.

after_commit callbacks are called on every record saved or
destroyed within a transaction immediately after the transaction is
committed. after_rollback callbacks are called on every record
saved or destroyed within a transaction immediately after the transaction
or savepoint is rolled back.

These callbacks are useful for interacting with other systems since you
will be guaranteed that the callback is only executed when the database is
in a permanent state. For example, after_commit is a good spot
to put in a hook to clearing a cache since clearing it from within a
transaction could trigger the cache to be regenerated before the database
is updated.

Caveats

If you're on MySQL, then do not use DDL operations in nested
transactions blocks that are emulated with savepoints. That is, do not
execute statements like 'CREATE TABLE' inside such blocks. This is
because MySQL automatically releases all savepoints upon executing a DDL
operation. When transaction is finished and tries to release
the savepoint it created earlier, a database error will occur because the
savepoint has already been automatically released. The following example
demonstrates the problem: